The present invention relates to a steering detection device for articulated vehicles which consists of drive means, or a tractor truck, which tows a semitrailer.
To allow medium and large articulated vehicles greater maneuverability, self-steering axles are currently widely used, that is to say, a pair of wheels with the relative supporting axle attached to the rear wheels, and which are self-steering when the lorry moves forwards.
Problems arise with these types of axles when the articulated vehicle must effect reversing maneuvers during which, on the free axle at the rear of the vehicle, understeering may arise, blocking the vehicle during the maneuver.
Two different solutions were examined in order to eliminate this fault on the self-steering axle. The first solution simply locks the axle in a set position, whilst the second, more technologically advanced, causes a controlled movement of the axle wheels even as the articulated vehicle or lorry reverses.
The first solution envisages that the self-steering axle be equipped with a locking device (mechanical--hydraulic), activated at the moment in which the reverse gear is engaged on the tractor truck, or more often by the driver (a pin is inserted in a corresponding hole made in the axle). The locking must be effected with the wheels in the normal forward position. However, this solution does not satisfy the maneuverability requirements of articulated vehicles, since not only is wear on the tires of the self-steering axle increased, but there are always parts which create friction on the blocked wheels, giving rise to errors in the steering direction of the articulated vehicle or lorry.
In contrast, the second solution, designed by the Applicant named herein (see patent application ITBO92A000264), envisages that the self-steering axle be equipped with a hydraulic drive unit (a twin action piston) which acts upon the axle wheels. In this solution, the unit is controlled by an electrical-hydraulic control unit, connected to the reverse control of the articulated vehicle or lorry so that it is activated only when the vehicle is put into reverse (that is to say, the axle remains passive during forward movement, and becomes active during reversing), and acts upon the piston chambers, allowing the self-steering axle wheels to move in the same way as the front wheels of the tractor truck. The synchronization of the tractor truck front wheels and those of the self-steering axle is controlled by a pair of detectors (linear transducers) which are connected to both steering axles and act upon the control unit which drives the self-steering axle. The use of transducers able to detect the steering angle of the controlled axle allows an exact reading of the effective angle (rather than the estimated angle). Thus, if the desired steering angle is not reached, for example due to obstacles to the wheels, upon removal of the impediment it does not prevent the wheels from positioning themselves at the said desired angle, since it is the transducer which "reads" the effective position and controls the hydraulic control unit.
This solution has proven to be technically reliable, although it has been noticed that while it is not difficult to fit linear transducers on normal lorries (on which they are attached to the axles of the directional wheels), on articulated vehicles of the tractor truck-semitrailer type the linear transducer which reads the tractor truck angle must be fitted between the tractor truck and the semitrailer.
The most suitable control point is the rotating restraint between the tractor truck and semitrailer (which replaces the lorry's front steering axle), which consists of a thrust bearing which acts as a joint between the tractor truck and semitrailer during maneuvers. However, technical difficulties prevent the fitting of the said linear transducer at this point, due both to its size (the transducer must be fitted in the extremely limited space between the thrust bearing and semitrailer), and to the fact that the angle of movement available to the tractor truck is, theoretically, much greater than that of the semitrailer and cannot be controlled by normal linear transducers.